One of conventional variable wavelength light source for optical communications system is a variable wavelength semiconductor laser device with an external resonator, such as the one disclosed in an article "Wide Tunable Wavelength and Narrow Linewidth Laser Diode Using External Grating" by Murakami et al. in the collections of articles presented at the 1988 Spring Conference of the Institute of Electronics and Communication Engineers of Japan.
FIG. 1 shows a basic arrangement of such a variable wavelength semiconductor laser device with an external resonator, which comprises a semiconductor laser element 1 which emits a light beam a from its front end surface and also a light beam b from its rear end surface. A collimator lens 2 collimates the light beam b. A diffraction grating 3 receives the collimated beam and reflects it. A stepping motor 4 rotates the diffraction grating 3 to alter the angle of the diffraction grating 3 relative to the incident collimated beam. The letter "c" denotes diffracted light produced by the diffraction grating 3.
In operation, the beam b emitted from the rear end surface of the semiconductor laser element 1 is incident on the collimator lens 2 in the form of a divergent light beam having divergence angle of about 10.degree.-45.degree. at intensities one-half the maximum intensity. The collimator lens 2 collimates the wavefront of the beam, and a collimated beam enters into the diffraction grating 3. When the collimated beam is incident on the diffraction grating 3, the grating 3 produces diffracted light. As is well known, there is predetermined relationship among the pitch of the grating, the wavelength of light, the incident angle of light, and the exiting angle of light. The exiting angle of diffracted light differs depending on the wavelength of incident light. Accordingly, by properly determining the above-described various conditions, such as the grating pitch and the incident angle of light, a light beam of desired wavelength selected from a plurality of incident light beams having different wavelengths can be selectively reflected back in the opposite direction to the incident light beam. This fact can be advantageously utilized. By rotating the diffraction grating 3 by means of the stepping motor 4, the angle of incidence of the light beam b onto the diffraction grating 3 can be varied so that a diffracted light beam c of a desired wavelength is selected by the diffraction grating 3 and reflected back in a direction 180.degree. opposite to the incident direction. The reflected diffracted light c passes the collimator lens 2 which changes the light c into a convergent beam incident on the rear end surface of the semiconductor laser element 1 (from which the beam b emerges). The beam c enters into a resonator of the semiconductor laser element 1 through this rear end surface which is provided with an anti-reflection coating so as to have a reflectance of 1% or less. The diffracted light c has a particular wavelength selected by the diffraction grating 3. Accordingly, light which has this selected wavelength is amplified by the resonator and oscillations at this single wavelength occur. Thus, the light beam a emerging from the front end surface of the semiconductor element has this selected wavelength. The wavelength of the light beam a can be changed by rotating the diffraction grating 3 by means of the stepping motor 4 to thereby change the angle of incidence of the beam b onto the grating 3.
The above-described conventional variable wavelength semiconductor laser device with an external resonator, therefore, requires a stepping motor and a power transmission system therefor, and, furthermore, the stepping motor and the power transmission system therefor require high precision. Accordingly, the conventional device has disadvantages that it is complicated in structure, large in size, and expensive.
Japanese Published Patent Application No. SHO 62-229890 discloses another type of variable wavelength light source device which comprises a combination of a semiconductor laser element and an ultrasonic wave deflector formed in a acousto-optical medium. In this device, output light from the laser element is collimated before it enters the ultrasonic wave deflector, and light emerging from the deflector at an angle which satisfies the Bragg condition is reflected by a planar reflector or an end surface of an optical fiber so as to travel back along the incidence path, but in the opposite direction from the incidence path, to the laser element. Thus, oscillation at a particular wavelength is sustained in the laser, and output light at this particular wavelength is derived. Selection or changing of the wavelength is made through selecting and changing the wavelength of ultrasonic waves in the ultrasonic wave deflector.
The device of this type does not include a mechanical driving member, and light at a particular wavelength can be selectively generated through electrical control of the device. However, the precision in selecting a desired wavelength is not satisfactory.
The present invention has been made to eliminate the above-described disadvantages. According to the present invention, a semiconductor laser device is free of any mechanical member for driving a diffraction grating which would otherwise be required for changing the oscillation wavelength of a semiconductor laser element and the oscillation wavelength can be selected electrically with improved precision. Thus, the device as a whole is simplified in structure and reduced in size. Further, the oscillation wavelength can be selected with high precision.